The invention relates to an electric hotplate, particularly for continuous operation in industrial or commercial use, with a hotplate body constructed as a solid body from iron material and particularly as a casting, which forms a cooking surface on its top and within a downwardly projecting outer flange edge is provided on the bottom with at least one electrical heating element distributed over an annular heating zone, as well as a lower seal below the heating element and comprising at least two superimposed sealing parts, whilst incorporating the outer flange edge.
In certain fields electric hotplates are subject to completely different operating conditions to those normally encountered when used in domestic kitchens and in normal operation. In the latter, the electric hotplate is generally heated from the cold state for so-called parboiling initially to a relatively high operating temperature, followed by finishing boiling for a period generally less than one hour at a reduced operating temperature. However, there are also electric hotplates intended to remain in continuous operation for several hours, e.g. from morning to evening and which are therefore not switched off during a normal working day. Such electric hotplates are used in the form of large hotplates in commercial kitchens, e.g. works canteens and the like. These electric hotplates, where there is a continuous operation, must be operable both at a low no-load temperature and under a favorable finishing boiling temperature in order to obtain a low power consumption, but must still be suitable to temporarily rapidly supply the necessary increased power in a heating phase interrupting the no-load or finishing boiling phase. The no-load power is e.g. required if no cooking utensils are located on the electric hotplate for a certain amount of time, but the hotplate cannot be switched off in order to maintain the necessary permanent operational readiness. The no-load power can also be used for finishing foods in the case of a relatively low power requirement.
Hitherto, the aforementioned requirements could not be satisfied in an optimum, simultaneous manner. Thus, in general an improvement to the no-load operation, both with regards to a reduction in the no-load temperature and as regards to the no-load power requirement has led to a detectable deterioration to the heating operation with respect to speed and vice-versa and this is particularly noticed when different sizes of cooking utensil are used.
An electric hotplate of the aforementioned type is e.g. known from EP-B No. 0 024 621. This electric hotplate, which is particularly intended for normal operation and is correspondingly designed, has as the lower seal incorporating the outer flange edge, a substantially closed sheet metal cover plate engaging on said flange edge and above the same a further sealing part in the form of insulation, which contains an aluminium crinkled foil secured between the cover plate and the outer flange edge. As this design is mainly provided for shielding the electric hotplate in the downward direction by insulation, a corresponding insulation increase would be necessary to increase the shielding effect, if this electric hotplate was to be made suitable for continuous operation.
However, those electric hotplates which, instead of a solid iron material body, have a hotplate body of glass ceramic or the like are less suitable for continuous operation due to their specific characteristics. With respect to severe operating conditions, electric hotplates made from an iron material have a much longer life and due to their thermal mass can better absorb briefly occurring temperature fluctuations. In addition, the thermal stresses occurring in the case of glass ceramic plates are of a completely different type to those occurring with hotplate bodies constructed in solid form from an iron material and which have an outer flange edge, whereby said hotplate bodies can be made either from steel or as a casting, e.g. from grey cast iron. Electric hotplates with glass ceramic plates, such as are e.g. known from U.S. Pat. Nos. 3,909,592, 4,032,750, 3,733,462 and 3,987,275 are admittedly provided with reflectors below the heating elements, but for the reasons indicated hereinbefore, quite different effects occur as compared with those with an electric hotplate of the initially mentioned type.